This invention relates to a carburetor for small engines, and more particularly to a carburetor having a fuel shut off solenoid device.
The use of solenoid devices to control a variety of fuel flow transients within a carburetor of a small engine is known. One particular application consists of a fuel shut off solenoid device of a carburetor capable of blocking fuel flow from entering a mixing passage of the carburetor when an ignition switch is turned off, thereby preventing engine dieseling and after boom. When the ignition switch is on, the solenoid device is energized and thereby held in a retracted position. If retracted, fuel flows from a fuel bowl, through a main tube or nozzle where the fuel premixes with air, and into the carburetor mixing passage to mix with more air. When the ignition switch is off, the solenoid device is de-energized and a head at a distal end of a shaft of the solenoid device is extended upward thereby isolating the fuel bowl from the main tube and effectively cutting off fuel flow.
The solenoid device is typically mounted in an upright position below the carburetor body. A solenoid chamber defined by an encasement of the device is usually disposed below the fuel bowl. When the head extends, the shaft of the solenoid device moves upward out of the solenoid chamber and the head mates with the bottom side of the main tube to cut off fuel flow. Because the solenoid chamber is located beneath the fuel bowl of the carburetor and a clearance exists between the shaft and the encasement of the solenoid device, fuel migrates via gravity into the solenoid chamber.
Although the migrating fuel was thought to be useful in cooling the energized solenoid device, it has been found that heat emitted by the coil of the energized solenoid device vaporizes the fuel contained within the solenoid chamber. The heat generated by the solenoid valve heats the fuel thereby creating vapor bubbles which migrate up through the main nozzle, interfering with steady or smooth operation of the engine. The bubbles interfere with the mixing of fuel and air causing a noticeably rough engine idling or light load condition. Accordingly, the present invention is a carburetor having a fuel shut off solenoid device which does not inject fuel vapor into the liquid fuel.
A carburetor body of the carburetor has an inner sidewall defining a mixing or lower chamber disposed above the fuel shut off solenoid device. A fuel chamber containing a constant level of fuel is defined by a fuel bowl engaged to an outward or underside of the carburetor body. Fuel flows through an orifice communicating between the fuel chamber and the lower chamber. The lower chamber communicates with an elongated main tube or nozzle defining an enriched fuel bore and extending longitudinally upward from the lower chamber and tranversely into a mixing passage. The main tube has a mating surface on a lower end facing downward and extending radially outward thereby engaging the sidewalls of the lower chamber.
The fuel shut off solenoid device has an encasement which threadably engages a bottom portion of the carburetor body. The encasement defines a solenoid chamber which houses an extendable shaft having a head at a distal end disposed above the encasement. The solenoid chamber contains migrating fuel thought to cool the solenoid coil. The shaft is disposed vertically within the solenoid chamber and extends upward through an outward face extending radially outward from an inner brim which circles the shaft. Mounted on top of the outward face and also circling the shaft, is a washer. When the solenoid is energized and in a retracted position, a head of the shaft engages to an outward face of the washer. The outward face of the encasement engages to the inward or opposite face of the washer.
The head of the shaft has an annular trailing surface expanding radially outward from an inner perimeter edge congruent to the surface of the shaft, to a peripheral edge. The trailing surface of the head confronts the outward face of the encasement. The peripheral edge of the trailing surface has a diameter larger than the diameter of a hole of the washer.
A clearance is defined radially between the shaft and the inner brim of the encasement. Fuel flows or migrates through the clearance between the lower chamber of the carburetor body and the solenoid chamber. While energized, the fuel shut off solenoid device has a tendency to heat the fuel in the solenoid chamber thereby creating vapor bubbles which can interfere with the idle or light load fuel mixture of the carburetor. The engagement of the washer between the trailing surface of the head and the outward face of the energized solenoid device partially blocks or stops the migration of fuel vapor bubbles from the solenoid chamber into the lower chamber.
Objects, features and advantages of this invention include the elimination of fuel vapors migrating from the solenoid chamber into the lower chamber of the carburetor body, a smoother operating engine, particularly noticeable during engine idling or light load conditions, and which is rugged, durable, economical to manufacture and assemble, and has a long useful service life.